The value of this number - known as the index "divisor" - is adjusted regularly to reflect index additions and deletions, share buybacks or issuances, spinoffs and other events that affect a company's market cap.

The basic idea appears to be use multiple running means to identify nodal points in sunspot data, and then to use a cubic fit between points so identified as a divisor for a mean somehow determined at each data point, although it is not quite clear to me what they did from what they have written!

He adds: "And we still divide that total to get the final number, but we use an index divisor, which is adjusted for stock splits and other corporate actions that might distort the continuity of the calculation."

"And we still divide that total to get the final number, but we use an index divisor, which is adjusted for stock splits and other corporate actions that might distort the continuity of the calculation," Mr. Prestbo adds.